Generally speaking, in the process of fabricating MOSFET in high density integrated circuit, polysilicon is often used as conducting wires between MOSFET elements to reduce the use of metal wires, therefore it is necessary to use buried contact structure. Conventional buried contact process is illustrated in FIGS. 1a to 1e which show the cross-sectional view of each step.
As shown in FIG. 1a, a field oxide 10 and a gate oxide 11 are grown on a silicon substrate 1. After a first polysilicon layer 12 and a first photoresist 13 are deposited, a buried contact region 14 is formed by defining the first photoresist 13 and etching.
As shown in FIG. 1b, the first photoresist 13 is removed to finish buried contact etch, then a second polysilicon layer 15 is deposited. Impurities could be implanted and driven in to improve the conductivity of the second polysilicon layer 15.
FIG. 1c shows a tungsten silicate (WSi.sub.x) layer 16 is deposited.
FIG. 1d shows that a second photoresist 17 is deposited and defined.
As shown in FIG. 1e, a buried contact region 18 is formed by anisotropic etching, and then the second photoresist 17 is removed.
Another conventional buried contact process is shown in FIGS. 2a to 2d. It is different from the above-mentioned process because tungsten silicate layer is not used. For convenience, corresponding elements use same numerals in all embodiments.
As shown in FIG. 2a, a field oxide 10 and a gate oxide 11 are grown on a silicon substrate 1. After a first polysilicon layer 12 and a first photoresist 13 are deposited, a buried contact region 14 is formed by defining the first photoresist 13 and etching.
As shown in FIG. 2b, the first photoresist 13 is removed to finish buried contact etch, then a second polysilicon layer 15 is deposited. Impurities could be implanted and driven in to improve the conductivity of the second polysilicon layer 15.
FIG. 2c shows that a second photoresist 17 is deposited and defined.
As shown in FIG. 2d, a buried contact region 18 is formed by anisotropic etching, and then the second photoresist 17 is removed.
The most often encountered problem of the conventional buried contact process is that although conducting wires are formed between MOSFET elements by polysilicon, if the misalignment of definition of the second photoresist is too large, the polysilicon layer 15 will not contact with the buried contact region 18 which leads to disconnection of wiring and therefore induces failure of the MOSFET elements.